1. Field of the Invention
The present invention relates to a diffusing agent composition, and a method for forming an impurity diffusion layer.
2. Description of the Related Art
According to the related art, an N-type impurity diffusion layer in a semiconductor substrate of a solar cell is formed by applying an diffusing agent that contains an N-type impurity diffusion component to a surface of the semiconductor substrate, and causing the N-type impurity diffusion component to diffuse from the applied diffusing agent. Specifically, a thermally oxidized film is first formed on a surface of a semiconductor substrate, and subsequently a photolithographic method is used to laminate a resist having a predetermined pattern onto the thermally oxidized film. The resist is used as a mask to etch, with an acid or alkali, regions of the thermally oxidized film that are not masked by the resist, and then the resist is peeled to form a mask of the thermally oxidized film. A diffusing agent which contains an N-type impurity diffusion component is applied thereto, thereby forming a diffusion film in regions in which openings in the mask are positioned. The regions are heated to a high temperature to cause the diffusing agent to diffuse into the regions, thereby forming an N-type impurity diffusion layer.
In recent years, suggestions have been made about methods of using an ink-jet method to pattern a surface of a semiconductor substrate with a diffusing agent into (see, for example, Japanese Patent Application Laid-Open JP-A No. 2003-168810, JP-A No. 2003-332606, and JP-A No. 2006-156646). In the ink-jet method, the diffusing agent is jetted from ink-jet nozzles to an impurity diffusion layer region for selective patterning. Thus, as compared with conventional photolithographic methods, the ink-jet method does not require any complicated step, and makes it possible to make a pattern easily while also reducing the amount of liquid to be used.
In conventional diffusing agents, phosphorus pentaoxide has been used as a source for supplying phosphorus, which is to be an N-type impurity diffusion component. In the application of a diffusing agent by a spin coating method, the concentration of a solid component (SiO2 or P2O5) can be set to a relatively low value; thus, a change of the solution used for the application with the passage of time is permissible. However, in the case of using an ink-jet method to pattern a semiconductor substrate surface with a diffusing agent in an attempt to save a painting solution to be used or to save process costs, a sufficient painted film thickness cannot be obtained unless the concentration of a solid component in the painting solution is high to some degree. However, in a case where a dehydration condensed product based on an Si compound is contained in the diffusing agent, the condensing reaction will progress rapidly when the solid component concentration in the painting solution is made high. As a result, there is caused a problem that the painting solution is remarkably deteriorated in storage stability.